As the AIDS pandemic worsens and the prospect for an HIV vaccine becomes dimmer, the need for effective HIV antiviral agents is more acute than ever. Yet the number of antiviral drugs is extremely limited. Moreover, treating chronic infections, especially in immunocompromised hosts, usually requires several agents that inhibit the pathogen at different steps of its life cycle. Novel antiviral agents, and anti-HIV agents in particular, must therefore be developed. While each step of the HIV life cycle is a potential site for therapeutic intervention, the fusion step is a particularly desirable target since it is the point at which the virus enters the cell. As a first step in rationally designing a specific HIV fusion inhibitor, the proposed project will analyze, at the molecular level, structural changes that occur in the hIV fusion inhibitor, the proposed project will analyze, at the molecular level, structural changes that occur in the HIV envelope glycoprotein as the virus fuses with a target cell. The strategy is modeled after studies that elucidated the fusion mechanism of the hemagglutinin of Influenza virus. A water-soluble HIV envelope glycoprotein ectodomain fragment that maintains native oligomeric structure will be produced to facilitate fusion studies. This form of the envelope glycoprotein will be analyzed for structural changes as it interacts with the receptor and target membrane. Methods will include monitoring alterations in antibody reactivity, protease sensitivity, and lipophilic properties. In particular, the project will test the hypothesis that the fusion domain of the envelope glycoprotein remains inaccessible to the target membrane, and therefore fusion inactive, until the envelope glycoprotein changes conformation by interacting with the target cell. Once the structural events required for fusion activity are identified, it will be feasible to rationally design an agent that locks the envelope glycoprotein in an inactive conformation, thereby preventing HIV infectivity.